1. The Invention
This invention relates to methods of melt-spinning fine denier polypropylene filaments using novel resin blends or pellets made from such blend.
2. Prior Art
The general melt-spinning of polypropylene into fibers is, of course, well known to the art; see, for example, U.S. Pat. Nos. 3,705,227; 3,233,023; 3,560,604; and 3,663,675.
The quality of polypropylene resin used to melt-spinning fine denier yarns (e.g. 40-600 denier/with 40 to 200 filaments per yarn and especially for 40-200 denier yarns) is very important. Generally, this quality refers to the size or molecular weight of the particular polymers composing the resin and the narrowness of the distribution of such polymers. Thus, a good spinning resin will in theory be composed of polymers of about the same size or molecular weight (or within a narrow range) whereas a poor spinning polymer will have a wide range of different sized polymers. In actual practice molecular weights are seldom referred to but rather the criterion for melt-spinnability is expressed in terms of "swell" and melt flow index. As used herein, the term "melt flow index" refers to the grams of polymer that flow through a specified orifice at a standard temperature and pressure; see ASTM D-1238-65T. The term "swell" or "swell index" refers to the ratio of the cross-sectional area of the extruded filament immediately after exit from the spinnerette to that of the spinnerette; see Journal of Applied Polymer Science, 18, No. 8, 2427, 1974. In practice it has been found that for melt-spinning good quality fine denier yarns the resin polymer (or the compounded resin polymer) should have a melt flow index of about from 30 to 38 and a swell of 2.8-3.4. If the "swell" is not great enough, the extruded filaments do not have the necessary elasticity and do not draw properly. It should also be noted that this quality requirement, although especially critical for spinning fine denier yarns, is not nearly so stringent for higher denier yarns.
A further problem is that the resin, whether supplied as a powder or pellet, is generally compounded with color pigments, etc. and then repelleted. The use of color pigments further reduces the "swell" and thus places even greater requirements on the quality of the initial resin.
Generally, polypropylene is sold in two principal grades or qualities. The first grade is referred to herein as C.R. (Controlled Rheological properties) and has been subjected to visbreaking, following polymerization, to narrow the molecular distribution and is generally intended to have melt flow indices of about 30 to 38 and a "swell" of around 3. In practice, however, the "swell" of this material is frequently below this and after compounding with pigments, is even lower. Frequently this problem is appreciated only after drawing difficulties are obtained, thus necessitating discarding the already spun fiber, or scrap recycling it, and using the remaining resins for less demanding uses. The melt flow index of the resin C.R. resin is generally adequate and insured by the prior art's use of about 0.05 to 0 weight percent, of a melt flow stabilizer.
The second or lower grade of polypropylene is referred to as reactor grade and has not been subjected to visbreaking and has a broad molecular weight distribution and generally undergoes extensive thermal degradation during melt-pelleting or melt-spinning. Reactor grade resin typically has a broad melt flow index, of about from 8 to 35, and generally has a fairly large swell of about 5 to 7.